The invention relates to a process for the production of a cam to be joined to a hollow shaft to form a camshaft in accordance with the precharacterizing clause of Patent Claim 1.
Weight-optimized camshafts can be produced economically by joining cams to a tube in a form- and force-locking manner. As regards functional reliability, the cams must have both a high adhesion on the tube for operational reliability even when accepting high engine torques and have a hardened track, and, for reasons of wear resistance, the latter must have sufficient internal compressive stresses after the joining of the cams. This applies particularly to the cam flank and, to a limited extent, also to the cam tip in valve timing systems with roller cam followers. Where internal compressive stresses are absent or internal tensile stresses are present in the cam track, additional tensile stresses are caused during engine operation by rolling-contact operations, and these stresses can lead to overloading of the material in the track, giving rise to fatigue or disintegration of the latter.
A method of the generic type is known from DE 44 20 C2.
The object on which the invention is based is to develop a method of the generic type in such a way that the cam is suitable for stable long-term operation on the camshaft, the internal compressive stresses achieved in the cam track before the joining of the cam to the camshaft being so high that the tensile stresses resulting from joining are permanently overcompensated.
According to the invention, the object is achieved by the features of Patent Claim 1.
Thanks to the invention, reproducible production of a cam with optimum functional properties with regard to secure retention of the cam on the hollow shaft under operational load after joining and to adequate intrinsic robustness in relation to the mechanical loads acting on it during engine operation is achieved by the interaction of a number of criteria. The selection of a cam material with a very high yield strength of at least 450 N/mm2 is absolutely decisive here for the achievement of the properties mentioned, these properties emerging from the heat treatment of the cam blank. Such cam materials include the hardenable steels 100 Cr 6, Ck 67, C 60, C 70 etc. To achieve the required minimum yield strength, these steels should be heat-treated before edge-zone hardening but can also be used in the hot-pressed condition (BY condition). The material stressed in accordance with the invention provides a high-strength structure which, owing to the large difference in yield strength relative to the tube, which is composed of the material St 37, St 52 or the like, for example, gives high cam adhesion during the subsequent joining of the fully-treated cam thanks to the elastic springback of the cam towards the tube, which is expanded plastically due to internal high pressure. In the case of heat treatment, it is necessary, in addition to making a suitable selection of steel material, to temper the steel in such a way that a basic hardness in a range of between 25 and 40 HRC is obtained since, below 25 HRC, unwanted plastic expansion of the cam during joining would occur and, above 40 HRC, stress cracks would occur in the cam track during subsequent edge-zone hardening or joining. The cam is then gently edge-zone-hardened by induction hardening in a two-stage heating process using an annular or shaped inductor at a medium frequency in a range of 10-35 kHz. Gentle edge-zone hardening means that the cam is preheated in the first stage of the heating process for up to 1.5 seconds at a power of about 40 kW, after which the introduction of heat is interrupted for 0.3-1.5 seconds. This pause avoids overheating effects in the structure, especially in the region of the track, and excessively high edge-zone hardening depths, which would normally occur after a second heating stage that immediately followed preheating, and a very gentle continuous drop in hardness from the edge zone to the core, which is close to the cam bore and is not edge-zone-hardened, is achieved. Such a drop in hardness is essential since otherwise the edge zone would be susceptible to cracking. The hardness in the region of the track is 60 HRC, for example, and decreases to 30 HRC in the region of the core. Moreover, the formation of a coarse martensite structure with soft residual austenite that occurs if further heating follows directly, i.e. if there is a second heating stage that continues directly after preheating, is avoided, leading to low internal compressive stresses in the structure.
Following the action of the preheat, the cam track is heated once more by means of medium frequency at increased power relative to that for preheating (second heating stage), giving a temperature above the austenitization temperature. The core structure is not affected by this process. Owing to the second heating stage in conjunction with the advantageous effects described of the preceding pause between the two heating stages of the heating process, it is now possible to establish an edge-zone hardening depth in the middle of the cam track of between at least 0.5 mm and at most 2.0 mm relative to the cam base circle and the cam flanks and at most 2.2 mm relative to the cam tip. It should be stated here that the figure of 0.5 mm in the middle of the track relates to a cam on which there is no need for a grinding allowance. Once the edge-zone hardening depth has been established and the heating process is complete, the cam is then immediately sprayed with an aqueous quenching medium (within a matter of milliseconds), as a result of which the austenitic structure of the cam is transformed completely into martensite. Very high internal compressive stresses arise in the structure during this transformation. The formation of other types of structure that produce only low internal compressive stresses is avoided owing to the rapid quenching to a temperature below the temperature for the formation of martensite. Moreover, the formation of a relatively thin edge zone during quenching prevents the outer regions of the edge zone from being transformed with a time offset relative to the regions closer to the core owing to the edge zone being too thick. If individual edge zone regions were to be transformed with such a time offset, the core zone, which is transformed latest, would produce tensile stresses in the cam track instead of internal compressive stresses. Almost simultaneous transformation of the cam structure in all regions of the edge zone is thus achieved according to the invention by the setting of a small edge-zone hardening depth and the immediate quenching of the cam, which can moreover be accomplished by means of oil or by means of a cold medium, such as liquid nitrogen. The complete transformation into a martensitic structure that takes place during this process generates very high internal compressive stresses in the cam owing to the expansion in the volume of the thin edge zone due to transformation, and these stresses overcompensate by far the tensile stresses that result from subsequent joining. In addition, internal compressive stresses at the surface of the track are produced by shrinkage processes in the cam during spraying.